Folylpolyglutamates are coenzymes in, and potential regulators of, a large number of reactions known collectively as one carbon (1-C) metabolism. These reactions which include the metabolic cycles for the synthesis of thymidylate, purines and the amino acids, methionine, serine and glycine, are compartmentalized in the mitochondria and cytosol of cells. This application is for the continuation of a series of studies aimed at investigating the control of the 1-C metabolism in cells and animals, and the role that mitochondrial folate metabolism plays in this process. The new application has five specific aims that are designed to test four hypotheses. The specific aims are: (1) to investigate the interrelationship between mitochondrial and cytosolic 1-C metabolism; (2) to study the regulation of 1-C entry and loss from the folate pool via the two compartmental forms of serine hydroxymethyltransferase; (3) to study the heterozygous disruption of the mouse methionine synthase gene and other genes for folate-dependent enzymes on the flux of 1-C units through the various metabolic cycles; (4) to investigate the use of the mouse methionine synthase heterozygous knockout as a model for the pathological and metabolic effects of vitamin B12 deficiency; and (5) to examine the regulation of expression of methionine synthase, methylenetetrahydrofolate reductase and serine hydroxymethyltransferase and to clone and characterize additional other genes of folate-dependent 1-C metabolism.